Structural sandwich panels having cores comprised of low density closed cell material, such as closed cell plastics foam material, and opposing skins comprised of fibrous reinforcing mats or fabrics in a matrix of cured resin, have been used for many decades in the construction of a wide variety of products, for example, boat hulls and refrigerated trailers. The foam core serves to separate and stabilize the structural skins, resist shear and compressive loads, and provide thermal insulation.
The structural performance of sandwich panels having foam cores may be markedly enhanced by providing a structure of fibrous reinforcing members within the foam core to both strengthen the core and improve attachment of the core to the panel skins, for example, as disclosed in applicant's U.S. Pat. No. 5,834,082. When porous and fibrous reinforcements are introduced into the closed cell foam core and a porous and fibrous skin reinforcing fabric or mat is applied to each face of the core, adhesive resin, such as polyester, vinyl ester or epoxy, may be flowed throughout all of the porous skin and core reinforcements by differential pressure, for example under a vacuum bag. While impregnating the fibrous reinforcements, resin does not saturate the plastic foam core because of its closed cell composition. The resin then co-cures throughout the reinforced structure to provide a strong monolithic panel.
It is desirable to produce sandwich panels of enhanced structural performance by improving the structural connections and support among reinforcing members within the foam core and between the core and the panel skins. This is desirable in order to resist buckling loads in the reinforcing members, to prevent premature detachment of reinforcing members from one another and from the skins under load, and to provide multiple load paths for the distribution of forces applied to the panel. Existing fiber reinforced core products offer important improvements over unreinforced foam in this regard but fail to integrate fully the separate reinforcing elements of the core into a unified and internally supported structure. For example, in a grid-like configuration of fibrous reinforcing sheet-type webs in which a first set of continuous webs is intersected by a second set of interrupted or discontinuous webs, the webs do support each other against buckling. However, under severe loading conditions, the discontinuous webs tend to fail at the adhesive resin bond to the continuous webs along their narrow line of intersection. This tendency may be substantially reduced by providing resin filled fillet grooves in the foam along the lines of intersection as disclosed in the above mentioned patent. Moreover, since the reinforcing fibers of interrupted webs terminate at each intersection with a continuous web, the structural contribution of those fibers is substantially less than that of the fibers of the continuous webs.
In the case of strut or rod type core reinforcements comprising rovings of fiberglass or carbon fiber or other fibers which extend between the faces of the core, individual struts within a given row of struts may intersect each other in a lattice configuration. This supplies buckling support to each strut, but only in the plane of the strut row. To achieve bi-directional support, struts of a first row must extend through the filaments of struts of an intersecting row. This requires difficult and costly levels of accuracy and control in machine processing, since all struts must be precisely positioned in three dimensions.